In the prior art of difference frequency mixing to produce infrared radiation, a four-photon process has been used to generate .omega..sub.4 = .omega..sub.1 + .omega..sub.2 - .omega..sub.3. (See, for example, U.S. Pat. No. 3,914,618, issued to Stephen E. Harris on Oct. 21, 1975). In that work the sum .omega..sub.1 + .omega..sub.2 was close to the frequency associated with a two-photon transition from an atomic ground state to an excited state. An example, taken from U.S. Pat. No. 3,914,618, is illustrated in FIG. 1, which shows the relevant energy levels of sodium. There, .omega..sub.1 and .omega..sub.2, corresponding to 6856 A, and the two-photon-transition is from the ground state of a sodium atom (3s) to the 3d state. If a beam of ultraviolet radiation at 3321 A is applied, output radiation at the difference frequency corresponding to a wavelength of 10.61 .mu. will be generated. In this and all other examples that have been found, the lower state of the two-photon transition, which is the lowest-energy state used in the mixing process, is the ground state of the conversion medium. This method of generating infrared radiation is energy-inefficient in the sense that two visible and one ultraviolet photon are required to produce every infrared photon.